Electrical properties of ceria-carbonate composite electrolytes

Electrical conductivity and transport properties for composites consisting of samaria-doped ceria (SDC) and lithium-potassium carbonate were investigated at temperature of 450-550 °C. The conductivity at temperature above 500 °C as measured with ac impedance spectrum is 0.06-0.07 S cm^-1, which is about one magnitude higher than that of SDC. The conductivity increases with carbonate content and is boosted to several times when the volume fraction of carbonate reaches 30%. An abrupt change in the electrical conductivity at 500 °C infers that superionic phase transition possibly occurs in the interface between SDC and carbonate. Negligible electronic conduction, ∼10^-4 S cm^-1 at 500-550 °C, is observed with an ion-blocking cell. The dc transport numbers of carbonate ion and proton are determined by carbon dioxide and vapor concentration cells, respectively. Carbonate ionic transport number is about 0.67 above 500 °C whereas the protonic transport number is below 0.1. The oxygen ion transport number is calculated to be 0.23 at 550 °C and 0.31 at 500 °C.